The Ovonic EEPROM is a novel, proprietary, high performance, non-volatile, thin film electronic memory device. In this device, information can be stored in either analog or binary form (one bit per cell) or in multi-state form (multiple bits per memory cell). The advantages of the Ovonic EEPROM include non-volatile storage, potential for high bit density and consequently low cost as a result of its small footprint and simple two-terminal device configuration, long reprogramming cycle life, low programming energies and high speed. There are small differences in the structure and the materials employed with these two types of memory devices, which differences enhance either the binary or multistate performance characteristics thereof. For purposes of the instant invention, the terms "memory elements" and "control elements" will be employed synonymously.
Before detailing the operation and structure of this novel and fundamentally unique Ovonic EEPROM device of the instant invention, some very important and pertinent thoughts concerning general semiconductor structures and operation will be discussed. The operation of most semiconductor devices is governed by the control of mobile charge carrier concentrations different from that generated at thermal equilibrium. Prior to the invention described in the above mentioned parent of the subject patent application, only four general methods were known by which to control and modulate the concentration of excess or free (these two terms are used interchangeably throughout this discussion) charge carriers in solid state semiconductor devices. These four known methods were previously described in said parent application, the disclosure of which is herein incorporated by reference, and need not be further discussed herein. However, a general discussion of those fundamental mechanisms of operation of semiconductor devices which are necessary in order to appreciate the advantages of the instant invention follows hereinafter.
In a perfect semiconductor lattice with no impurities or lattice defects--an intrinsic semiconductor--no charge carriers are present at zero Kelvin since the valence band is filled with electrons and the conduction band is empty. At higher temperatures, however, electron-hole pairs generated as valence band electrons are excited thermally across the band gap to the conduction band. These them ally generated electron-hole pairs are the only charge carders present in an intrinsic semiconductor material. Of course, since the electrons and holes are created in pairs, the conduction band electron concentration (electrons per cubic centimeter) is equal to the concentration of holes in the valence band (holes per cubic centimeter). It is well known, but worth emphasizing, that if a steady state carrier concentration is to be maintained, there must be recombination of the charge carriers at the same rate that they are generated. Recombination occurs when an electron in the conduction band makes a transition to an empty state (hole) in the valence band, either directly or indirectly through the agency of a mid-gap recombination center, thus annihilating the pair.
In addition to thermally generated charge carders, it is possible to create carders in semiconductor materials by purposely introducing certain impurities into the crystal lattice. This process is called doping and represents a common method of varying the conductivity of semiconductors. By doping, a semiconductor material can be altered so that it has a predominance of either electrons or holes, i.e., it is either n-type or p-type. When a crystal lattice is doped such that the equilibrium carrier concentrations are different from the intrinsic carrier concentrations, the semiconductor material is said to be "extrinsic". When impurities or lattice defects are introduced into an otherwise perfect lattice crystal, additional levels are created in the energy band structure, usually within the band gap. For instance, the introduction of phosphorous in silicon or germanium, generates an energy level very near the conduction band. This new energy level is filled with electrons at zero Kelvin, and very little thermal energy is required to excite these electrons to the conduction band. Thus, at about 50-100 Kelvin, virtually all of the electrons in the impurity level are donated to the conduction band. Semiconductor material doped with donor impurities can have a considerable concentration of electrons in the conduction band, even when the temperature is too low for the intrinsic charge carrier concentration to be appreciable. Now that the reader can appreciate the significance of the presence of excess charge carriers for electrical conductivity, it must be noted that these carriers can also be created by optical excitation or they can be injected across a forward biased p-n junction or a Schottky barrier. Regardless of the manner in which the excess carriers are generated, they can dominate the electrical conduction processes in a semiconductor material.